1. Field of the Invention
This invention relates to the field of molten metal processing and handling. More specifically, the invention relates to an improved method and apparatus for preheating pouring tubes and nozzles, such as submerged entry nozzles for a continuous casting machine.
2. Description of the Related Technology
Production of metals by use of the continuous casting technique has been increasing since its large-scale introduction about thirty-five years ago, and now accounts for a large percentage of the volume of steel, among other metals, produced each year worldwide. Continuous casting machines typically include a mold that has two essentially parallel and opposed wide walls, and two essentially parallel opposed narrow walls that cooperate with the wide walls to define a casting passage of rectangular cross section. Molten metal is supplied continuously into a top end of the casting passage, and the mold is designed to cool the metal so that an outer skin forms before the so-formed slab or strand exits a bottom of the casting passage. Spraying further cools the strand as it travels away from the mold, until it becomes completely solidified. It may then be processed further into an intermediate or finished metal product, such as steel plate, sheeting or coils by traditional techniques such as rolling.
In conventional continuous casting machines, molten steel is teemed from a ladle into a tundish that typically has a one or more of holes defined in its bottom, each of which is connectable to at least one removable submerged entry nozzle (SEN), which is constructed and arranged to guide the flow of molten steel from the tundish into the continuous casting mold. Special refractory slide gate valves and/or stopper rods are usually provided to control the flow of molten metal to and through the submerged entry nozzle. In addition to the submerged entry nozzle, other refractory pouring tubes are associated with the ladle and the tundish, such as ladle shrouds which are employed to protect the molten metal from ambient oxidation during the teeming/casting operations, and nozzles to guide the molten metal through the brick lining the bottom of the tundish. All of these refractory shapes, which may be referred to herein as molten metal contacting elements, are subjected to severe operating conditions and must be able to withstand thermal shock, as well as the chemical/erosive attack of molten steel and slag.
Molten metal contacting elements are commonly made from carbon containing compositions, including one or more refractory grains such as alumina, zirconia, clays, magnesia, silicon carbide, silica or other dense grains having a specific mesh size. These refractories also generally contain significant amounts of carbon in the form of graphite, carbon black, coke and like carbon sources plus a carbonaceous binder derived from sources such as pitch or resin. Such pressed and fired refractory shapes are known to possess good physical properties, particularly thermal shock, making them suitable for use in this severe operating environment.
Molten steels are commonly de-oxidized or "killed" by the addition of aluminum metal, ferromanganese or ferrosilicon. In the common case of aluminum killed steel, the added aluminum will react with dissolved oxygen or iron oxide to form finely dispersed aluminum oxide in the melt, some of which remains as highly dispersed microparticles in the solidified steel while a portion floats into a layer of slag that floats above the molten steel. During continuous casting, this extremely finely dispersed portion of alumina has a tendency to either precipitate out of the molten steel onto the cooler refractory surfaces or react with and bind to the refractory surfaces. This gradual build-up of alumina causes problems in the control of the flow of molten steel and may eventually cause blockage in the pouring nozzles. In addition, molten metal contacting elements such as submerged entry nozzles wear quickly due to factors such as friction and oxidation, and need to be replaced relatively often.
Before a submerged entry nozzle can be replaced, the new nozzle must be preheated in order to minimize thermal stress within the nozzle during start-up and to prevent the molten steel from solidifying within the nozzle before it reaches the mold. Conventionally, this preheating is performed with gas burners, which are directed inside the ports of the nozzle so that the combustion takes place within the nozzle itself. This method of heating takes from 30 to 60 minutes and has the undesired effect of causing oxidation in the inner working surface of the nozzle. It further has the effect of burning off or otherwise weakening the carbonaceous binder within the refractory material, which will ultimately shorten the working life of the nozzle. Elemental carbon is susceptible to air oxidation at temperatures above 500 degrees C. Since submerged entry nozzles often must be replaced on an unscheduled basis as the continuous casting machine operates, it is common for many steelmakers to keep a new submerged entry nozzle at the ready by the continuous casting machine in a constant preheated state to avoid expensive delays. This practice has several environmental and economic drawbacks. First, keeping a new nozzle preheated consumes an enormous amount of energy, which is not recoverable by the steel producer and tends to end up as waste heat that is introduced into the environment. Second, a new nozzle will only remain useful for a limited period of time while it is being kept in the preheated state, before excessive oxidation and binder degradation takes place. As a result, many new nozzles are discarded without ever being used, which represents a significant expense to the steel producer. Others are downgraded to a shorter life expectancy that is commensurate with the amount of extra time the nozzle remains in the preheated state.
A need exists for an improved method and system for preheating molten metal contacting elements such as submerged entry nozzles that is more energy efficient and environmentally and economically sound than methods and systems that have heretofore been known and used.